1. Technical Field
The present invention relates to the generation of pulse sequences for use in telephone signalling and, more particularly, to circuitry for improving the reliability of pulse sequences in telephones wherein the pulses are generated by key operated calling devices, and, more particularly, for those telephones that include circuitry requiring a memory function, e.g., last number redial, and repertory dialing.
2. Background Art
Since the invention of electronic telephones that simulate the mechanical switching action of the typical rotary dial telephone, a number of manufacturers have designed circuits that provide dial pulses, a muting function and sometimes memory and other control functions. Such circuits have recently been implemented as integrated circuits; MOSTEK MK50981 and MK5175 units being typical of such devices. Numerous other manufacturers also have provided similar integrated circuit units. These devices have proven to be reliable except under certain conditions where the devices can be subject to temporary loss of memory or false output of data. One cause of such malfunction is found to be due to transients fed back through the external switching components into the integrated circuit. These transients may be external to the telephone or they may be the result of the telephone dial pulses under certain conditions, such as may be present with extra high telephone line voltage, etc. Previously utilized techniques to overcome the outlined problem have included the use of a varistor or other surge protection device connected between the positive and negative supply lines to the integrated circuit. Such varistor units are used to protect against surges, such as a lightning surge or other gross fault condition. However, a better protection method for the pulse feedback problem and other type of transients is required. Accordingly, it is the object of the present invention to improve the reliability of operation of the telephone, especially as related to problems caused by pulse feedback. cl SUMMARY OF THE INVENTION
The present invention consists of an electronic telephone, including a key operated pulse generating calling device integrated circuit, and includes a diode clamp applied to the integrated circuit pulse generating device output and connected with its anode to the associated power supply common lead. Also included is a Schottky diode in the pulse signal output circuit to a network switching transistor. Each diode provides a certain amount of protection on its own to the included circuitry and in combination provide maximum protection. The first diode insures that no voltage in excess of approximately 1 volt negative will appear on the integrated circuit pulse output terminal, due to the diode clamping action. The second diode in the pulse signal path to the transistor switch blocks negative current transients in the signal path, essentially blocking negative pulse feedback.
In operation, the protective devices can be thought of as one-way valves permitting the flow of positive current but shunting and blocking, respectively, negative transient currents. In operation, when the integrated output goes to a "high logic" condition (a positive voltage of approximately 1 volt) signalling the first of two transistors to an "on" state, the first diode does not interfere and the second diode allows conduction, to bias the final switching transistor to an "off" state, disconnecting the voice network in the telephone to produce the dial pulse. When the network is disconnected, inductive effects will generate a negative voltage on the switching transistor that would otherwise travel into the signal path and back into the integrated circuit. The Schottky diode blocks the negative transient. If any voltage still gets through to the first diode from the integrated circuit pulse output, the diode will shunt the remaining voltage to common, thereby by-passing the internal circuitry of the integrated circuit. The inclusion of the Schottky diode is also found to improve the alternating current impedance of the pulse network.